In processes such as just mentioned, the lactam can be recovered from the resulting reaction mixture of lactam and sulfuric acid in a known way (see, e.g., U.S. Pat. No. 2,993,889) by neutralizing the sulfuric acid in the reaction mixture by means of ammonia water, thus forming ammonium sulfate, after which the lactam can be separated from the resulting ammonium sulfate. This method has, however, the disadvantage that a large amount of ammonium sulfate is obtained as a by-product, which does not always find a ready market.
According to U.S. Pat. No. 3,336,298, this disadvantage can be obviated by omitting the aforesaid neutralization of the sulfuric acid in the reaction mixture, and, instead, converting the sulfuric acid into ammonium hydrogen sulfate by the addition of ammonium sulfate, and then separating the lactam from the ammonium hydrogen sulfate by extraction. The remaining by-product will then be a concentrated aqueous solution of ammonium hydrogen sulfate, which can be commercially used, e.g., in the decomposition of phosphate rock. While it is true that no ammonium sulfate is obtained as a by-product in this last-mentioned reprocessing method, the recovery of the desired lactam is still always coupled with the preparation of another product in a large quantity.
A method for the recovery of the lactam without the formation of by-products has been described in U.S. Pat. No. 3,852,272. In this method, the sulfuric acid is only partially neutralized with ammonia water and, after extraction of the lactam from the partially neutralized mixture, an aqueous solution containing ammonium hydrogen sulfate remains which is then subjected to a decomposition treatment to form sulfur dioxide. This sulfur dioxide is then used to make the sulfuric acid needed for the formation of the reaction mixture of lactam and sulfuric acid. A disadvantage of this method is, however, the loss of ammonia in the decomposition of ammonium hydrogen sulfate, since the ammonia in the ammonium hydrogen sulfate is fully burnt to nitrogen and water.
U.S. Pat. No. 3,879,380 also describes a method for the recovery of lactam in which no by-products are obtained, but in which no ammonia is burnt. In this method, the sulfuric acid is partially neutralized, i.e., to form an ammonium-hydrogen-sulfate melt, after which ammonia and sulfur trioxide are recovered separately from this melt by means of a suitable metal oxide, such as, e.g., zinc oxide. However, the recovery of ammonia and sulfur trioxide in this way is very expensive.
To improve these known processes, it has already been proposed (see non-prepublished Netherlands Patent Application No. 7,601,061, the disclosure of which is incorporated herein by reference), to decompose thermally the ammonium salt obtained in the complete or partial neutralization of the sulfuric acid so that a gaseous mixture containing ammonia and sulfur dioxide is formed. This gaseous mixture is then brought into contact with the reaction mixture of lactam and sulfuric acid. The ammonia will then be bound to the sulfuric acid, while a sulfur dioxide-containing gas is discharged. Netherlands non-prepublished application No. 7,601,061 was published Aug. 5, 1977 and corresponds to U.S. Pat. No. 4,081,442 issued Mar. 28, 1978.
According to still another proposal for improving the known processes (see non-prepublished Netherlands Patent Application No. 7,607,047, the disclosure of which is incorporated herein by reference), part of the ammonium hydrogen sulfate obtained in the partial neutralization of the reaction mixture of lactam and sulfuric acid is subjected to thermal decomposition, to form a gaseous mixture containing ammonia and sulfur dioxide, and this gaseous mixture is brought into contact with the remaining part of said ammonium hydrogen sulfate, whereby ammonia is bound and a sulfur dioxide-containing gas is discharged. The ammonium sulfate thus formed with the bound ammonia is then used for the partial neutralization of the reaction mixture of lactam and sulfuric acid. Netherlands non-prepublished application No. 7,607,047 was published Dec. 30, 1977 and corresponds in turn to U.S. application Ser. No. 810,568 filed June 27, 1977.
The sulfur dioxide gas thus obtained in the aforesaid proposed improvements can be in turn converted in sulphuric acid, oleum or sulfur trioxide to be used for instance in the rearrangement of cyclohexanone oxime to caprolactam.
It has now been found that the resulting sulfur dioxide-containing gas as such, is less than entirely suitable for the catalytic oxidation to sulfur trioxide. This is because of the reducing agent that has to be present in said thermal decomposition, e.g., the combustion products of oil or natural gas, so that the sulfur dioxide concentration is rather dilute in the resulting gas. Moreover, this gas contains compounds that have an adverse effect on the catalyst used for the oxidation of sulfur dioxide to sulfur trioxide.
It is therefore the essential object of the present invention to provide a process whereby a gas containing sulfur dioxide can be obtained that is more suitable for the catalytic conversion thereof to sulfur trioxide.